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The story of the three quake- and tsunami-hit reactors at Japan's Fukushima plant continues, with indications that one of the three worst-hit reactors has sustained further damage. A fire also broke out at another reactor, shut down at the time of the quake and not previously thought to be a problem, but this has now been put out. None of this suggests that the reactors' crucial containment vessels could be breached, however.

World Nuclear News reported in the early hours (UK time) that a "loud noise" had been heard at the site of the No 2 reactor at Fukushima and pressure readings had fallen in the doughnut-shaped "suppression chamber" situated beneath the core. The suppression chamber holds a large quantity of water and steam from the core, released into it to be condensed, so reducing pressure in the core without the need to vent to atmosphere in some situations: though reactors 1, 2 and 3 have all been repeatedly vented to atmosphere since the quake nevertheless.

Following the apparent release from inside the suppression chamber, radiation levels at the site briefly rose to 8217 microsieverts per hour – such that an unprotected person outside would receive several years' normal background radiation dose in a single hour. Radiation then dropped to less than 2,500 microsievert/hr. (UPDATED TO ADD: The IAEA reports that as of 6am UK time this had fallen to 600 microsievert/hr). WNN reports that a statement from TEPCO, the plant operators, stated that there had been "no significant change" to the status of the vital containment vessel surrounding the reactor core.

According to nuclear experts at MIT, much of the radiation from steam releases comes from shortlived isotopes created when neutrons from the core strike water: these have half-lives of seconds, and are safe within a minute of being emitted. This is why previous high levels of radiation and continual steam releases within the plant site have not caused major health concerns for local residents on the part of the Japanese government.

However, as the crisis has developed, the normal coolant – pure demineralised water – has been replaced at the stricken reactors by seawater pumped in using the site's firefighting systems. Salt and other contaminants in the seawater result in the creation of other, more dangerous isotopes – though in small amounts. Furthermore it has been obvious since the weekend – and since officially admitted – that water levels in the reactors have sunk below the level of the fuel rods on several occasions, meaning that the fuel rods will have sustained heat damage and so that some material from their alloy cases or even the uranium itself could be present in the steam escaping from the reactors.

The probability of steam escaping directly from No 2's suppression chamber could increase levels of radiation further, as normal, controlled releases are carried out via filters and scrubbers. TEPCO said it was removing all non-essential personnel from the site as a precaution.

Thus far however, indications are that longer-lived isotopes are not being released from the plant in dangerous quantities. The Japanese authorities have nonetheless moved to complete a previously-ordered evacuation out to 20km from the plant site and that those 20-30km out should remain indoors to minimise risk from inhaling airborne radionuclides or getting them on their skin.